1. Field of the Invention
This invention relates to the field of information networks, and more particularly relates to a method for allocating bandwidth in a network.
2. Description of the Related Art
Today's networks carry vast amounts of information. High bandwidth applications supported by these networks include streaming video, streaming audio, and large aggregations of voice traffic. In the future, these bandwidth demands are certain to increase. To meet such demands, an increasingly popular alternative is the use of lightwave communications carried over fiber-optic cables. The use of lightwave communications provides several benefits, including high bandwidth, ease of installation, and capacity for future growth.
The synchronous optical network (SONET) protocol is among those protocols employing an optical infrastructure. SONET is a physical transmission vehicle capable of transmission speeds in the gigabit range, and is defined by a set of electrical as well as optical standards. SONET's ability to use currently-installed fiber-optic cabling, coupled with the fact that SONET significantly reduces complexity and equipment functionality requirements, gives local and interexchange carriers incentive to employ SONET. Also attractive is the immediate savings in operational cost that this reduction in complexity provides. SONET thus allows the realization of a new generation of high-bandwidth services in a more economical manner than previously existed.
SONET networks have traditionally been protected from failures by using topologies that dedicate something on the order of half the network's available bandwidth for protection, such as a ring or mesh topology. Two approaches in common use today are diverse protection and self-healing rings (SHR), both of which offer relatively fast restoration times with relatively simple control logic but do not scale well for large data networks. This is mostly due to their inefficiency in capacity allocation. Their fast restoration time, however, makes most failures transparent to the end-user, which is important in applications such as telephony and other voice communications. The existing schemes rely on 1-plus-1 and 1-for-1 topologies that carry active traffic over two separate fibers (line switched) or signals (path switched), and use a protocol (Automatic Protection Switching or APS), or hardware (diverse protection) to detect, propagate, and restore failures.
A SONET network using an SHR topology provides very fast restoration of failed links by using redundant links between the nodes of each ring. Thus, each ring actually consists of two rings, a ring supporting information transfer in a “clockwise” direction and a ring supporting information transfer in a “counter-clockwise” direction. The terms “east” and “west” are also commonly used in this regard. Each direction employs it's own set of fiber-optic cables, with traffic between nodes assigned a certain direction (either clockwise or counter clockwise). If a cable in one of these sub-rings is damaged, the SONET ring “heals” itself by changing the direction of information flow from the direction taken by the information transferred over the failed link to the sub-ring having information flow in the opposite direction.
The detection of such faults and the restoration of information flow thus occurs very quickly, on the order of 10 ms for detection and 50 ms for restoration for most ring implementations. The short restoration time is critical in supporting applications, such as current telephone networks, that are sensitive to quality of service (QoS) because it prevents old digital terminals and switches from generating red alarms and initiating Carrier Group Alarms (CGA). These alarms are undesirable because such alarms usually result in dropped calls, causing users down time aggravation. Restoration times that exceed 10 seconds can lead to timeouts at higher protocol layers, while those that exceed 1 minute lead to disastrous results for the entire network. However, the price of such quickly restored information flow is the high bandwidth requirements of such systems. By maintaining completely redundant sub-rings, an SHR topology requires 100% excess bandwidth.
An alternative to the ring topology is the mesh topology. The mesh topology is similar to the point-to-point topology used in internetworking. Each node in such a network is connected to one or more other nodes. Thus, each node is connected to the rest of the network by one or more links. In this manner, a path from a first node to a second node uses all or a portion of the capacity of the links between those two nodes.
Networks based on mesh-type restoration are inherently more capacity-efficient than ring-based designs, mainly because each network link can potentially provide protection for fiber cuts on several different links. By sharing the capacity between links, a SONET network using a mesh topology can provide redundancy for failure restoration at less than 100% of the bandwidth capacity originally required. Such networks are even more efficient when traffic transits several links. One study found that for an 11-node, 22-span network, only 51% redundant net capacity was required for 100% restorability, as reported in, “The design and simulation of an intelligent transport network with distributed control,” by T. Chujo, H. Komine, K. Miyazaki, T. Ogura, and T. Soejima, presented at the Network Operations Management Symposium, San Diego, Feb. 11–14, 1990, which is incorporated herein by reference, in its entirety and for all purposes. The corresponding ring-based design required five rings and a total DS-3 redundancy of 330%. However, path restoration often consumes several minutes in such a topology. This is much slower than the restoration times exhibited by ring topologies and is so long that connections are often lost during the outage.
Various kinds of networking equipment can be used to support the ring and mesh topologies just described. Options include:                1. Back-to-back wavelength division multiplexers (WDMs) and optical cross-connects (OXCs) for use in mesh topologies.        2. Back-to-back optical add/drop multiplexers (O-ADM) for ring topologies.        3. Other combinations (e.g., WDM combined with OXC, digital cross-connect systems (DCSs), and other such equipment)        
WDMs may be connected in back-to-back configurations to allow the connection of various wavelength routes to one another (also known as “patching” or “nailing up” connections). Provisioning paths in such architectures is done manually using a patch panel. Thus, provisioning is slow and prone to mistakes due to human error and equipment failure. In the event of a failure, restoration is performed manually in such architectures and is again slow and error-prone. Such architectures scale poorly because additional bandwidth is added by either adding to the number of wavelengths supported (requiring the replacement of equipment at nodes, and possibly the replacement of fiber-optic cables as well) or adding new fiber-optic cables and supporting node equipment. Such architectures are also inherently unmanageable, due to the lack of centralized control. And, while the initial capital investment tends to be relatively low (as a result of their simplicity), operating expenses for such architectures tend to be relatively high because of the costs associated with configuration, expansion, and management. Thus, a mesh topology employing back-to-back WDM's will tend to be slow to deploy and difficult to manage due to the need for manually “nailing up” paths and lack of centralization.
Another architectural element that may be used to create a mesh topology is the optical cross-connect (OXC). OXCs allow provisioning using a centralized scheme to accomplish provisioning in a matter of minutes. Restoration in the event of a failure may be performed manually or may be effected using a centralized management system. However, restoration still requires on the order of minutes per wavelength route restored. As with the back-to-back WDM architecture, a mesh topology that employs OXCs scales poorly. This is due in part to the exponential increase in the physical size experienced when expanding the capacity of an OXC with the addition of input and output links. For example, an OXC that supports two links (fiber-optic cables), each having three paths, will need to provide a switching fabric that supports the six possible combinations of connections between the paths carried by the two fiber-optic cables. When this number is increased to four paths per fiber-optic cable, the number of possible connections increases to twenty-four. As still more paths are added to each link and more links are supported, the possible number of connections increases dramatically, increasing the physical size of the affected OXC.
An OXC can be either transparent (i.e., purely optical, in which the signals are never converted to electrical signals) or opaque (i.e., the optical signals are converted into electrical signals and then converted back into optical signals). Transparent optical cross-connects provide little in the way of manageability because the information carried by lightwave is never made accessible to the OXC's operator. In contrast, opaque OXCs can be configured to permit access to the information being switched. However, neither type of OXC maintains information regarding the topology of the network and, in fact, OXCs possess no intrinsic network intelligence. Moreover, OXC technology is expensive, making initial investment quite high, as well as the cost of future expansion.
Alternatively, a SONET network may be configured in a ring (SHR) topology by using add/drop multiplexers (ADMs). An ADM is a SONET multiplexer that allows DS 1 signals to be added into or dropped from an STS-N signal. ADMs have two bidirectional ports, commonly referred to as an east and a west port. Using ADMs, a SONET network in a SHR topology uses a collection of nodes equipped with ADMs in a physical closed loop such that each node is connected to two adjacent nodes with a duplex connection. Any loss of connection due to a single failure of a node or a connection between nodes is automatically restored. The traffic terminated at a failed node, however, is lost. Two types of SHRs are unidirectional (USHR) and bidirectional (BSHR), as defined by the traffic flow in normal conditions. Bidirectional rings have a capacity carrying advantage over unidirectional rings because of the ability to share protection capacity among the links between nodes, as opposed to unidirectional rings, which dedicate capacity all the way around the ring.
Provisioning in such architectures is centralized and can be performed in minutes. While restoration can also be performed quickly (on the order of 50 ms, as previously noted), 100% spare bandwidth is required. Thus, the user must install fiber-optic cabling for two networks, one for normal traffic and one to be used in the event of a failure. Moreover, the cabling for each link should be physically located as far from its corresponding link in order to minimize the possibility that a cause of physical damage will damage both links and cause both directions of a ring to fail. These issues detrimentally affect cost, manageability, and scalability. With regard to expansion, ADMs are stacked in an SHR in order to increase capacity. However, stacked ADMs are blocking. In other words, the switching function may not allow the transfer of data from a port on one stacked ring to a portion on another ring. Thus, an architecture employing ADMs is best suited for small offices or other situations that do not require the relatively large amounts of bandwidth (implying the need for stacked ADMs). As noted, stacked ADMs are also difficult to manage and expensive due to the extra hardware required for 100% spare capacity.
Other combinations can also be employed. For example, WDMs can be combined with OXCs (either transparent or opaque) in order to create a network having a mesh topology. Such an architecture supports the cross-connection of wavelength routes by either manual connection or under centralized control. However, such an architecture is also difficult to expand due to the need to add WDMs/fiber-optic cables and the increase in size of the OXC, and cannot restore failed links quickly enough to avoid dropping or interrupting telecommunications connections.
Another option is the use of a digital cross-connect system (DCS). A DCS is used to terminate digital signals and cross-connect them, integrating multiple functionalities such as signal adding and dropping, cross-connection capabilities, and multiplexing and demultiplexing of signals. DCS based networks enjoy an advantage over networks employing back-to-back WDMs because the use of DCS eliminates the need for additional back-to-back electrical multiplexing, thus reducing the need for labor-intensive jumpers. Operational cost savings are realized by a DCS through electronically controlling cross-connections, test access and loopbacks, and maintenance. Two types of DCSs are wideband DCSs and broadband DCSs. Wideband DCS (W-DCS) terminates full duplex OC-Ns and DS3s, has VT cross-connection capability, and provides DS1 interfaces. A broadband DCS (B-DCS) terminates full-duplex OC-N signals and provides, for example, STS-1 and DS3 interfaces. The B-DCS makes two-way cross-connection at the DS3, STS-1, and concatenated STS-Nc levels. STS-Nc may be used, for example, in broadband services such as high definition television (HDTV), where an STS-3c cross-connection may be used to cross-connect the signal as a single, high-capacity channel.
Various attempts have been made to use DCSs in a mesh configuration to create a fault-tolerant network, but none have been successful in reducing restoration times below a few seconds. Some of these configurations rely on a central database and a central controller (usually an Operations System or OS) to restore failures. Although these schemes often exhibit restoration times exceeding 10 minutes, such restoration times are an improvement over manual restoration, which requires hours, or even days to effect restoration. However, these results are not enough to meet the 50–200 ms restoration time required by existing telecommunication network equipment. Other implementations employ distributed architectures in which control is shared among multiple network nodes. This results in faster restoration times (on the order of about 2–10 seconds), but still does not address the need for restoration times below 200 ms.